1. Field of the Present Disclosure
The present disclosure relates to an image processing apparatus and an image forming apparatus.
2. Description of the Related Art
An image forming apparatus has an area separating function that identifies a property of each area in an image such as a character area, an image area, or the other areas. The image forming apparatus performs the most suitable process for each property to gain the image quality of an area having each property.
Such an image forming apparatus performs printing of a character area using a single color of Black (K), and performs printing of an image area either using three colors of CMY (Cyan, Magenta, and Yellow) to express vivid colors (i.e. chromatic colors with middle or high luminance) or using four colors of CMYK with a black generation and UCR (Under Color Removal) process to express dense colors (i.e. chromatic colors with low luminance).
However, the area separating function sometimes identifies a wrong property. For example, in some cases an edge part in a bold character is identified as a character area but an inner part of the bold character is identified as an image area. In such cases, the image processing of the character edge part and the inner part uses different settings therebetween, and consequently results in a density difference between the character edge part and the inner part.
Further, in some cases, even in a gradation image, some partial pixels are not identified as an image area but identified as a character area. In such cases, the gradation image is partially printed with a single color of Black, and consequently strong granular feeling may appear in the image.
An image processing apparatus (referred as the first technique) changes a black generation amount in accordance with the smallest value among color component values of CMY in an input signal and a difference between the largest value and the smallest value.
Further, another image processing apparatus (referred as the second technique) identifies a hue in a CMY input signal, and changes a black generation amount in accordance with the identified hue.
Furthermore, another image processing apparatus (referred as the third technique) distinguishes a black character's inner part and a black character's edge and identifies a density of the black character's inner part; and for the inner part of a high density bold character, performs a different process from a process for the character edge, and for the inner part of a low density bold character, performs a similar process or a relatively near process to a process for the character edge.
Furthermore, another image processing apparatus (referred as the fourth technique) converts input RGB data into the HLS color space and changes the data to an achromatic color if a chroma saturation of the data falls into a predetermined area in the HLS space; and changes an input gradation level as the maximum black generation amount when the black generation is performed on the basis of the input data.
However, in the aforementioned first and third techniques, the image processing apparatuses do not use hue information and therefore do not perform proper black generation corresponding to a hue.
Further, in the aforementioned second technique, the image processing apparatus sometimes reads a CMY input signal of an inner part of a bold black character as not an achromatic color but a chromatic color with low luminance, and in such a case, the inner part of a bold black character still has low image quality.
Furthermore, in the aforementioned fourth technique, the image processing apparatus increases the black generation amount only for a black character, and therefore a large difference of the black generation amounts may appears between a part identified as a black character in an image area and the other part and consequently granular feeling may be emphasized in a gradation image.
As mentioned, in the aforementioned techniques, a black character still has low image quality caused by incorrectly identifying an inner part of the black character as an image area.